The technical field of this invention is induction motor controls. In recent years the advent of modern power electronics has substantially changed the motor and controller hardware used in traction control or electric vehicle applications. DC motors have been very popular in such applications in the past because of their characteristic high torque at zero speed and because the decoupled field and armature currents allow independent control of either the field flux or the output motor torque and provide fast response. However, DC motors have the disadvantages of lower machine efficiencies and the high maintenance that accompanies the use of brushes.
The disadvantages of DC motors can be eliminated by the use of an AC induction motor with an IGBT inverter. AC induction motors are robust since they use no brushes or commutators and are capable of four quadrant operation using a variable frequency variable voltage drive control. The use of vector control techniques allows the flux producing component and torque producing component of motor current to be decoupled to produce a motor response like that of a DC motor.
Vector control for induction motors is generally classified as indirect vector control (IVC) or direct vector control (DVC). For high performance industrial motor control and electric vehicle applications, indirect vector control using a rotor shaft speed encoder is becoming very popular. With indirect vector control, an induction motor can be operated at any point in the torque-speed plane, from zero speed to the field-weakening region. However, the motor parameters may vary, which causes flux-torque coupling problems and degrades control. In addition, this method requires a speed sensor on the motor shaft and feed-forward slip frequency signals for the required generation of unit vectors. The speed sensor is undesirable because it is expensive and can be unreliable.
Direct vector control generates the unit vectors from flux coils or from the motor terminal voltages and currents, and no speed sensor is required. Direct vector control with stator flux orientation can reduce the problems caused by varying motor parameters; but it has been difficult to operate the motor near zero speed, and self starting of the motor is impossible. The lower speed range in direct vector control has recently been extended by using stator flux orientation and an observer method of flux computation. The motor could be started by a volts/hertz scalar control and then converted into direct vector control; but this start-up method provides inferior performance.